There is known a substrate processing system that performs etching by soaking a substrate in processing liquid in a processing bath, for removing part of films on the substrate and forming a desired pattern, or removing all of the films (e.g., refer to Patent Literature 1). In a manufacturing process of a semiconductor device, out of a silicon nitride film (Si3N4 film) and a silicon dioxide film (SiO2 film) that are formed on a substrate such as a silicon wafer, the silicon nitride film is often selectively eliminated through etching. A phosphoric acid (H3PO4) aqueous solution is often used as processing liquid for removing the silicon nitride film.
Due to its properties, the phosphoric acid aqueous solution etches not only the silicon nitride film but also a slight amount of the silicon dioxide film. Because minute patterns are required of today's semiconductor devices, it becomes important to keep an etching rate constant for controlling an etching amount, and to keep a selection ratio, which is a ratio between respective etching rates of the silicon nitride film and the silicon dioxide film, constant. As a technique of keeping the etching rate and the selection ratio constant, for example, there is known a method of periodically adding a certain amount of pure water to processing liquid while heating the processing liquid so as to bring the processing liquid into a boiling state.